This application is based on Application No. 10-239804 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a spectral image input device, and specifically relates to a spectral image input device which inputs an interference image of an object to a optical element using a optical element having birefringence characteristics.
2. Description of the Related Art
Conventionally, to obtain a spectral interference image of an object, the interferometer principle is generally used. An interferometer typically divides a light ray into two optical paths using a beam splitter or the like, and after inducing a slight optical path difference (phase difference) and unifying the rays, utilizes the generated interference. FIGS. 8 and 9 show an image input device which uses the interferometer principle. In FIGS. 8 and 9, reference number 1 refers to an image forming lens, reference number 2 refers to a beam splitter, reference numbers 3 through 6 refer to total reflective mirrors, and reference number 7 refers to an area optical element element. In this type of device, the mirrors are moved along the optical axis and rotated to produce an interference image of the light at different wavelengths, thereby functioning as a spectral image input device.
On the other hand, interference devices are known which do not divide the light rays into two optical paths. This method uses materials having a birefringence. In this interference device, a birefringence material having a principal refractive index axis (crystal axis) of 45xc2x0 relative to a polarizer is disposed between an analyzer and a deflector arranged at a mutual polarization angle of 90xc2x0. The birefringence material has different refractive indices for ordinary rays and extraordinary rays, and the speeds at which the ordinary rays and the extraordinary rays pass through the material also differ. For these reasons a phase difference is generated by the ordinary rays and the extraordinary rays transmitted material of identical thickness. When the phase difference is an integer multiple of a single phase period of light of a certain wavelength, the light of this wavelength generates interference. FIG. 10 illustrates the generation of a phase difference by a material having birefringence; Reference symbol A represents the principal refractive index axis direction, reference symbol B represents oscillation direction and speed of the incidence rays, reference symbol Bo represents the oscillation direction and speed of ordinary rays passing through the material, and reference symbol Be represents the oscillation direction and speed of extraordinary rays passing through the material.
Conventional interference devices are known to use liquid crystals as materials having birefringence. FIG. 11 shows an example of such an interference device; reference number 11 refers to a polarizer, reference number 12 refers to an analyzer, and reference number 13 refers to a liquid crystal, which is interposed between glass plates 14. The arrow A indicates the principal refractive index direction of the liquid crystal. FIG. 12 shows an example wherein a plurality of liquid crystals 13 are arranged so as to change the wavelength of the light generating interference when a voltage is applied between the glass plates 14. FIG. 13 shows an example wherein two wedge shaped liquid crystals 13 are overlaid such that the principal refractive index axis directions A are perpendicular, and interference of light of different wavelengths is generated by the position at which the rays pass through the liquid crystals.
The interference device using a beam splitter shown in FIGS. 8 and 9 are disadvantageous, however, inasmuch as a long optical path is required to generate interference, and a specialized optical system is necessary. Furthermore, the device itself is enlarged as the optical path is lengthened.
On the other hand, interference devices which use material having a birefringence are advantageous insofar as the optical path can be shortened. However, material having a birefringence has a difference in the refractive indices of the ordinary rays and the extraordinary rays on the order of 1/1000, e.g., a thickness of approximately 60 xcexcm is necessary to accurately realize interference of light at a wavelength of 590 nm. High precision can be realized providing glass plates on bilateral sides of a liquid crystal. It is difficult to arrange the principal refractive index axis when the liquid crystal is a relatively thick 60 xcexcm. When considering that the thickness of the liquid crystals typically used in display devices is at most several micrometers, it becomes extremely difficult to make a practical interference device using liquid crystals. Liquid crystals also require fluid leakage countermeasures.
An object of the present invention is to provide an improved spectral image input device.
Another object of the present invention is to provide a spectral image input device having a short optical path and compact construction, and is capable of producing an interference image at a predetermined wavelength.
Still another object of the present invention is to provide a spectral image input device which can be manufactured and assembled with excellent precision, and is capable of producing an interference image at a predetermined wavelength.
These objects are attained by providing a spectral image input device comprising, an image forming optical system for forming images of image light on an image forming surface; and an optical image element formed of a solid material having a birefringence and disposed anterior to the image forming surface in the direction of travel of the image light. This optical element provides a phase difference in accordance with the wavelength of the image light based on the difference in physical thickness of the optical element in the optical axis direction.